The body core temperature is of special interest when measuring the inner temperature of objects, especially of the human body. Fields of application are medical engineering during the monitoring of adults, children and newborns in the intensive care unit and safety engineering, in general, personal safety and for members of firefighter teams.
A device of this type is known from DE 10 2005 004 933 B3 (corresponding to U.S. Pat. No. 7,299,090). In the prior-art device, the temperature of the skin surface is measured with a first temperature sensor, and a second temperature sensor, which is arranged at a spaced location from the first temperature sensor via a heat insulation, detects the temperature near the environment. Taking the heat transfer coefficient of the tissue of the living being and the heat transfer coefficient of the insulation into account, the body core temperature of the living being can be calculated from the measured temperatures. The idealized formula for the calculation is based on the assumption that the heat flux released by the skin surface onto the temperature-measuring device is sent completely from the first temperature sensor to the second temperature sensor.
By combining the skin temperature with the heat flux, which is obtained from the difference of the two temperatures, the body core temperature Tcore is then calculated as:
                              T          core                =                                            T              1                        +                                                            k                  s                                                  k                  g                                            ⁢                              (                                                      T                    1                                    -                                      T                    2                                                  )                                              =                                                    T                1                            ·                              (                                  1                  +                                                            k                      s                                                                                                                                  ⁢                                              k                        g                                                                                            )                                      +                                          T                2                            ·                                                k                  s                                                                                                          ⁢                                      k                    g                                                                                                          (        1        )            Here, T1 denotes the temperature of the first temperature sensor near the body and T2 the temperature of the second temperature sensor away from the body. Factor ks is the heat transfer coefficient of the insulator between the temperature sensors and kg is the heat transfer coefficient of the human tissue between the body core and the first temperature sensor near the body. The two temperatures of the first and second temperature sensors are linked with one another linearly in the formula.
A heat flux due to energy loss, which is released to the sensor housing of the temperature-measuring device, does additionally occur in a real system. To take the heat flux due to energy loss into account, a marginal temperature sensor is provided, which is arranged in the area of the outer wall of the sensor housing and detects the marginal temperature of the sensor housing at the transition to the environment. The taking into account of the heat flux due to energy loss leads in the formula used for the calculation to a compensation term, which depends on the measured marginal temperature. The drawback of this is that the marginal temperature sensor can detect the marginal temperature only locally and effects from the environment may distort the measurement. The problem is compounded by time-dependent environmental effects, which lead to a time-dependent correction.